All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
A group of scholars stated the permissibility of placing the old person whose mental capacity has changed under Hajr (Interdiction to use one's own property due to some Sharee'ah reasons) because such a person – like the insane and the demented person – does not know well how to manage his affairs and spend money. ‘Abdur-Razzaq narrated in his Musannaf that: “Najdah wrote to Ibn ‘Abbaas asking him about the ruling on the senile, old man. Ibn ‘Abbaas informed him that the senile man should be placed under Hajr.”
Hence, if your father’s condition is as you described in your question, you should treat him well and be dutiful to him in matters that conform to reason. You should temporarily prevent him from disposing of his property. You should do so wisely and kindly until the sharia court judges on his case whether he should be placed under Hajr or not.
You should hasten to submit the case before such a court because it alone is entitled to issue a judgment concerning this issue. If it places him under Hajr, then you are not considered undutiful to him in case of implementing the judgment. However, you always have to be kind and gentle in dealing with him.
Allaah Knows best.